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Medical Virology Lecture 29

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Medical Virology Lecture 29 Tasnim Suliman MSc Molecular Virology (student) 2535939_at_uwc.ac.za What are viruses? Small obligate intracellular parasites Virion Complete ... – PowerPoint PPT presentation

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Title: Medical Virology Lecture 29


1
Medical VirologyLecture 29
  • Tasnim Suliman
  • MSc Molecular Virology (student)
  • 2535939_at_uwc.ac.za

2
What are viruses?
  • Small obligate intracellular parasites
  • Virion
  • Complete virus particle nucleic acid protein
    coat, which may be surrounded by an envelope
  • It is the form in which the virus moves between
    cells or hosts
  • Viral Genome
  • EITHER RNA or DNA genome surrounded by a
    protective virus-coded protein coat (Capsid)
  • Propagation depends on specialized host cells
    supplying the machinery for replication,
    metabolism and biosynthesis

3
  • The DNA or RNA genome may be
  • ss single stranded or
  • ds double stranded
  • Genomes may be either
  • () sense Positive-sense viral RNA is identical
    to viral mRNA and thus can be immediately
    translated into protein by the host cell.
  • OR
  • (-) sense Negative-sense viral RNA is
    complementary to mRNA and thus must be converted
    to positive-sense RNA by an RNA polymerase before
    translation.
  • Retroviruses?

4
Definitions
  • Bacteriophage
  • Virus that infects prokaryotic (bacterial)
    cells.
  • Nucleocapsid
  • viral nucleic acid the protein coat that
    encloses it.
  • Represents the packaged form of the viral genome.

5
Viral Structure - Overview
Spike protein
Fig 1. Schematic overview of the structure of
animal viruses does not exist in all viruses
6
Viral Structure
  • Varies in size, shape and symmetry
  • VIP for classification
  • 3 types of capsid symmetry
  • Cubic (icosahedral)
  • Has 20 faces, each an equilateral triangle. Eg.
    adenovirus
  • Helical
  • Protein binds around DNA/RNA in a helical fashion
    eg. Coronavirus
  • Complex
  • Is neither cubic nor helical eg. poxvirus

7
http//micro.magnet.fsu.edu/cells/virus.html
8
Viral Structure
Figure 1 An array of viruses. (a) The helical
virus of rabies. (b) The segmented helical virus
of influenza. (c) A bacteriophage with an
icosahedral head and helical tail. (d) An
enveloped icosahedral herpes simplex virus. (e)
The unenveloped polio virus. (f) The icosahedral
HIV with spikes on its envelope.
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10
Viral Replication
  • When a virus infects a cell, nucleic acid must be
    uncoated and gain access to metabolic machinery
    of cell.
  • Virus life cycle is characterized by
  • attachment
  • penetration, with entry of nucleic acid into cell
  • early expression of virus genes (either directly
    by translation, if virus contains "" RNA, or
    indirectly after transcription and then
    translation)
  • replication of virus nucleic acid
  • synthesis of new virion components
  • packaging and assembly of new virions
  • exit from cell

11
  • Attachment
  • specific binding of a virion protein (the
    anti-receptor) to a constituent of the cell
    surface (the receptor)
  • e.g. hemagglutinin of influenza virus
  • some complex viruses (HSV) may have more than one
    species of anti-receptor molecule
  • Penetration
  • energy-dependent step
  • occurs almost instantaneously after attachment

12
  • After the virus attaches to the host cell, it can
    enter the cell by several mechanisms
  • Transfer of the entire viral particle across the
    cell membrane by endocytosis
  • Transfer of only the viral genome through the
    cell membrane
  • Fusion of the viral envelope with the host cell
    membrane

13
  • Uncoating
  • at same time as penetration or shortly after
  • separation of viral nucleic acid (n.a.) from
    outer structural components
  • Released as
  • free n.a. (picornaviruses)
  • as nucleocapsid (reoviruses) may need acidic pH
    in endosome
  • viruses only infectious agent for which
    dissolution of infecting agent obligatory step in
    replicative pathway
  • Expression of viral genome and synthesis of viral
    components

14
  • After the viral nucleic acid is released inside
    the host cell
  • The transcription and translation processes of
    the host cell are redirected for the production
    of viral proteins and nucleic acids
  • The different types of nucleic acid genomes are
    expressed and replicated in several ways
  • DNA genomes undergo replication-using processes
    similar to cellular replication
  • RNA genomes may be ssRNA Can be read directly
    as an mRNA or reverse transcribed by reverse
    transcriptase into DNA
  • RNA genomes may also be -ssRNA The RNA must
    first be used as a template to form mRNAs

15
Assembly and Release
  • Components of capsid synthesis directed by late
    genes
  • Assembly of enveloped viruses needs interaction
    with plasma membrane which has been modified
  • Final stage of infection
  • Enveloped viruses released gradually by budding
    or exocytosis
  • Naked viruses accumulate in cytoplasm and
    released during lysis

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Viroids Prions
  • Viroids
  • ss RNA genome and the smallest known pathogens.
  • Affects plants
  • Prions
  • Infectious particles that are entirely protein.
  • No nucleic acid
  • Highly heat resistant
  • Animal disease that affects nervous tissue
  • Affects nervous tissue and results in
  • Bovine spongiform encepahltits (BSE) mad cow
    disease,
  • scrapie in sheep
  • kuru Creutzfeld-Jakob Disease (CJD) in humans

19
For more info
  • http//virology-online.com/general/Replication.htm
  • http//www.hhmi.org/biointeractive/animations/infe
    ction/inf_middle_frames.htm (animation)
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